This application relates to determining switching characteristics in electronic devices.
Various electronic devices operate between two states. For example, a two state magnetic memory device can be constructed from magnetic materials using multilayer structures which have at least one ferromagnetic layer configured as a “free” layer whose magnetic direction can be changed between two states by an external magnetic field or a control current. Magnetic memory devices may be constructed using such multilayer structures where information is stored based on the magnetic direction of the free layer.
One example for such a multilayer structure is a magnetic or magnetoresistive tunnel junction (MTJ) which includes at least three layers: two ferromagnetic layers and a thin layer of a non-magnetic insulator as a barrier layer between the two ferromagnetic layers. The insulator for the middle barrier layer is not electrically conducting and hence functions as a barrier between the two ferromagnetic layers. When the thickness of the insulator is sufficiently thin, e.g., a few nanometers or less, electrons in the two ferromagnetic layers can “penetrate” through the thin layer of the insulator due to a tunneling effect under a bias voltage applied to the two ferromagnetic layers across the barrier layer.
The resistance to the electrical current across the MTJ structures varies with the relative direction of the magnetizations in the two ferromagnetic layers. When the magnetizations of the two ferromagnetic layers are parallel to each other, the resistance across the MTJ structures is at a minimum value RP. When the magnetizations of the two ferromagnetic layers are anti-parallel with each other, the resistance across the MTJ or SV is at a maximum value RAP. The magnitude of this effect is commonly characterized by the tunneling magnetoresistance (TMR) in MTJs defined as (RAP−RP)/RP.
The MTJ can be placed in either the parallel or antiparallel resistance states through the application of a magnetic field to the device, or through the application of a current through the device via the spin transfer effect.
One technique used to measure switching characteristics in an electronic device, such as, for example a MTJ structure being switched via the spin transfer effect, is by application of a ramped series of electrical pulses. For example, as depicted in FIG. 1, the device under test (DUT) 110, e.g. the MTJ, might be placed in series with a known resistor 120. The application of a ramped series of electrical pulses might produce a voltage waveform 210 across the device under test as depicted in FIG. 2.
The state of the device may then be determined for the device at each pulse and after each pulse with the application of a low bias electrical pulse in order to determine the switching characteristics. FIG. 3 plots the resistance of a device, here a MTJ, against the ramped pulse voltage. In this example, the voltage pulses begin at −160 my and increase in amplitude. The resistance 310 is measured during the applied electrical pulses and the resistance 320 is measured after each electrical pulse with a low bias electrical pulse. The device switches from the low resistance state into the high resistance state at the application of a critical voltage, e.g., −370 my in this example. In addition, the resistance measured during the ramped electrical pulses decreases with increasing voltage magnitude due to the functional dependence of MTJ resistance on voltage. The noise in measured resistance is lower during the application of the ramped electrical pulse verses the low bias electrical pulse due to the larger input voltage.